Actors and products in species concepts

Wayne Maddison & Jeannette Whitton

Biological diversity is not a continuous cloud of variation, but rather appears strongly
bundled into clusters of organisms notably alike in their genes, structures and ecology.
This is enough to justify the working taxonomist's pragmatic approximations of species,
but to play fruitful and precise roles in biology, these units need to match important
components in our evolutionary or ecological models — they need to be actors in, or
notable products of, nature's processes. For example, in our models of evolutionary
process, a genetic community (e.g., an interbreeding population) acts as a unit, having
stochastically-predictable behaviour that produces particular patterns of genetic
diversity. In phylogenetic biology, minimal monophyletic groups are products of evolution
that serve as building blocks for our historical reconstruction. These (and other
process-relevant units) yield concepts of species that can each play a central role
in our understanding of how life functions or is structured, but each might lead to
different delimitations of groups of individuals. The degree of this discordance is
poorly characterized, but to the extent that it arises from differing interests of
biologists, it is not easily resolved.

There is a second sense of discordance: contradictions among patterns pertinent to
a single species concept. This is especially evident in concepts that define species
as outcomes of an evolutionary process, either as fragments of a realized history
(e.g., minimal monophyletic groups), or stepping even further toward operationalism,
as sets of individuals sharing features. Incomplete lineage sorting can cause different
genetic loci to show contradictory patterns as they descend within genetic communities,
yielding a discordance among genes that makes simple pattern-based species concepts
either ambiguous or arbitrary. Under alternative concepts that view species as actors
in a model of process, such tangled outcomes may complicate our inference, but they
do not necessarily compromise the discreteness of the species themselves — discrete
evolutionary actors can produce tangled patterns.

Should species be actors in a model of process (i.e., the causes), helping to explain
our observations of phenotypic diversity, or should they be the products (i.e., the
effects), to be explained by our models? Regardless of our decision, the varied (and
important) concepts can be used regardless, with different labels, in their respective
domains. Applying the taxonomist's pragmatism to the theorists' modelling, one could
seek a single umbrella species concept with the largesse to accommodate several more
precise ones, but the units so defined would likely not serve precisely enough in
models and explanations, either as actors or products.